Infrared spectroscopy for determining the concentration of individual gas components has previously been described. The most common methods relate to the Fourier transform infrared spectrometer or the non-dispersive infrared spectrometer. With the development of compact high-power semiconductor lasers, gas analyzers based on the laser spectroscopy have been established to an increasing extent. New laser types, such as quantum cascade lasers, have revolutionized laser spectroscopy in the medium infrared range.
All of the above analyzing methods rely on specific frequency ranges being absorbed during irradiation of a sample gas with infrared beams. The infrared radiation lies in the range of the oscillation level of the molecular bonds which are induced to oscillate by the absorption. A prerequisite therefor is a dipole moment which is already present or which is generated in the molecule. The different oscillation states cause absorption losses of the infrared radiation of different optical frequencies. The spectrum in the transmission thus contains individual absorption lines characteristic of the gas so that the sample gas can be examined for the presence of specific molecules, and their concentration in the sample gas can be determined.
A quantum cascade laser can in particular determine pollutant molecules present in the exhaust gas of an internal combustion engines, such as dinitrogen monoxide, nitrogen monoxide, nitrogen dioxide, carbon dioxide, carbon monoxide, and ammonia, and their concentration.
Common laser-spectroscopic systems comprise a laser as a radiation source, the radiation of which is conducted into a gas cell via an optical path. The beam is repeatedly reflected in the gas cell via a suitable mirror configuration. A sample gas flow is at the same time introduced into the gas cell, wherein the radiation of the laser penetrates the sample gas flow and excites the molecules corresponding to the optical frequency. Energy of the respective frequency is absorbed due to this excitation, and the intensity of the transmitted beam decreases at this location in the spectrum. The absorption itself is not exactly defined, but is subject to broadening due to temperature and pressure changes. The beam having its spectrum changed in this manner exits the measuring cell and impinges upon a detector via which the changed frequency band is evaluated, thus allowing the presence of specific substances and their concentration to be determined. The sample gas flow is usually fed via a downstream vacuum pump.
When the concentration is determined, the absorption characteristic in the spectrum is evaluated and/or analyzed. This characteristic is generally referred to as the line spectrum of the absorbing gases. It has turned out, however, that the line shape in this spectrum depends on pressure and temperature. For the purpose of evaluation, these parameters must therefore either be kept constant or must be continuously metrologically detected and offset. The gas is therefore conditioned and the pressure and the temperature kept constant in order to increase measuring accuracy.
It has also turned out that in particular during the measurement of hot and wet gases, such as exhaust gases of internal combustion engines, condensate formation in the analysis cell must in any case be prevented since the condensate leads to a considerable falsification of the measuring results; the measuring temperatures must therefore frequently be increased. It has also turned out that cross sensitivities can be avoided with decreasing pressure since the absorption spectrum at a negative pressure becomes very narrow and high, whereby the spectra of the individual components no longer overlap each other. The analysis cells are therefore operated at a negative pressure which amounts, for example, to approximately 200 hPa absolute pressure.
It is therefore common practice to perform a feeding of the measurement gas via vacuum pumps. In the case of analyzers having quantum cascade lasers as a radiation source, this is usually performed by a membrane pump or a rotary vane pump.
These pumps are, however, disadvantageous in that they generate pressure bursts which result in pulsations in the feed line, which, again, has a negative effect on the quality of the measuring results if these pulsations are not corrected using additional components. Membrane pumps can also normally only be operated at an ambient temperature of up to 40° C. and are further constrained with regard to the temperature of the feed gas and/or high costs are incurred when a higher temperature resistance is required. Rotary vane pumps have a relatively high weight which makes them difficult to integrate into the housing of an analyzer. Both pump types also require regular maintenance and suffer from increased wear.
DE 10 2006 05 901 therefore describes an analyzer where the feeding of the gas is performed by a suction jet pump which is largely maintenance-free since it does not comprise any movable parts. The regulation of the feed pressure is effected via regulating valves arranged in the feed line.
The disadvantage of such a regulation is an increased propellant gas consumption since feeding must be carried out against the resistance of the throttle in the feed line so that a high propellant gas pressure always exists.